docs: def list_subnets(self, identifier=None, datacenter=None, version=0,
                       subnet_type=None, network_space=None, **kwargs):
          """Display a list of all subnets on the account.
  
          This provides a quick overview of all subnets including information
          about data center residence and the number of devices attached.
  
          :param string identifier: If specified, the list will only contain the
                                      subnet matching this network identifier.
          :param string datacenter: If specified, the list will only contain
                                      subnets in the specified data center.
          :param int version: Only returns subnets of this version (4 or 6).
          :param string subnet_type: If specified, it will only returns subnets
                                       of this type.
          :param string network_space: If specified, it will only returns subnets
                                         with the given address space label.
          :param dict \\*\\*kwargs: response-level options (mask, limit, etc.)
          """
          if 'mask' not in kwargs:
              kwargs['mask'] = DEFAULT_SUBNET_MASK
  
          _filter = utils.NestedDict(kwargs.get('filter') or {})
  
          if identifier:
              _filter['subnets']['networkIdentifier'] = (
                  utils.query_filter(identifier))
          if datacenter:
              _filter['subnets']['datacenter']['name'] = (
                  utils.query_filter(datacenter))
          if version:
              _filter['subnets']['version'] = utils.query_filter(version)
          if subnet_type:
              _filter['subnets']['subnetType'] = utils.query_filter(subnet_type)
          else:
              # This filters out global IPs from the subnet listing.
              _filter['subnets']['subnetType'] = {'operation': '!= GLOBAL_IP'}
          if network_space:
              _filter['subnets']['networkVlan']['networkSpace'] = (
                  utils.query_filter(network_space))
  
          kwargs['filter'] = _filter.to_dict()
          kwargs['iter'] = True
          return self.client.call('Account', 'getSubnets', **kwargs)